Reentry is an abnormality of impulse conduction wherein an excitatory wavefront circulates around an inexcitable region. Figures 16.5 and 16.6 show a normally propagated and a reentrant event in injured ventricular myocardium, respectively. As illustrated in Figure 16.5, the wave of excitation passes through homogeneous tissue involving the Purkinje system (Pj and P2) and enters normal ventricular myocardium. As indicated in the figure, the wave of excitation conducts around an inexcitable barrier, collides within the tissue, and extinguishes within the ventricular myocardium. A normally propagating impulse will enter ventricular myocardium nearly simultaneously at multiple regions where Purkinje fibers terminate in the walls of both ventricles. The sequence of activation of the ventricular myocardium is rapid (~0.04 second). The net result is orderly activation of all ventricular myocardial fibers, giving rise to normal-appearing action potentials in the respective regions and a normal electrocardiogram.

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